Electrodes employing such electrode holders are available in two basic configurations. According to the one configuration, the electrode consists of two axially aligned sections, i.e. the electrode holder, constituting the upper section and comprising a cooled metal shaft, and at its lower tip, an active part of consumable material from which the electric arc eminates. This type of electrode is generally known as combination electrode. With a second configuration, the active part of consumable material is axially movable within the electrode holder, generally a cooled metal shaft. As consumable material of the active part is consumed at the electrode tip, compensation can be effected by axial movement of the active part. Such electrodes are generally known as conventional feed-through electrodes. A common criterion of both configurations is that the electrode holder, i.e. the liquid-cooled metal shaft, during operation, projects at least partly into the interior of the furnace.
Electrodes for electric arc furnaces are exposed to powerful thermal and mechanical stress. Thermal stress conditions result from the elevated electrode working temperatures characteristic of the production of electric steel. Mechanical stress is caused for example when the electrode contacts scrap material, or may result from movements of molten metal or scrap material within the furnace, or from vibrations related to the electric arc.
In order to ensure the usefulness of these electrodes it is, therefore, imperative to protect effectively the metal shaft of the electrode holder, which during operation is confined within the interior of the furnace, against these stresses. Numerous solutions have been offered to this problem.
One combination electrode holder described in BE-PS No. 867 876 takes the form of a metal shaft containing the cooling system and surrounded by a coating resistant to elevated temperature on the outside. In order to improve the adhesive capacity of this coating on the sheath area, the metal shaft has hooks configured for holding the coating in place.
Similar combination electrodes and holders are described in GB-PS No. 1 223 162, completely covered with a protective ceramic coating. With these ceramic coatings, attention to the thickness of the ceramic coating is necessary, which should be as small as possible, and to the extent to which such a coating is applied including the electrode holder proper thereby ensuring the insulation of coolant pipes therein. These pipes serve not only as cooling water ducts but also as current supply to the active graphite electrode part.
European patent application No. 0 010 305 describes a combination electrode with an electrode holder comprising a metal shaft electrically insulated from a current-carrying cooling system and which can be sufficiently cooled through material resistant to elevated temperature interposed between the cooling system and the metal shaft. The lower section of the metal shaft, which constitutes the electrode holder, is covered with a ceramic coat that is also secured by hooks.
The combination electrode of DE-AS No. 27 25 537 has a metallic, liquid-cooled upper section constituting the electrode holder, which is insulated by a material resistant to high temperature covering thermally conductive projections. The purpose of these projections is to prevent a direct mechanical contact with the line system if, as a result of high local stress, the high-temperature resistant material is locally damaged by scrap material. At the same time these projections perform as a kind of fuse, thereby preventing excessive currents from passing.
Finally, DE-AS No. 27 30 884 describes a conventional feed-through electrode having a cooled metal shaft, constituting the electrode holder and serving as a passage through which the active part of graphite is fed, covered with high-temperature resistant material. At the same time the metal shaft has projections directed radially towards the outside which fastenly engage a material resistant to high temperature. These projections, which are distributed along the periphery and in an axial direction as evenly as possible, appear configured to ensure a more constant cooling and better adhesion of the material resistant to high temperature to the shaft. This solution corresponds to the protective coat designs mentioned in connection with combination electrodes. According to the most recent state of technology, the same solutions are offered for the electrode holders of combination electrodes and conventional electrodes.
All these electrode holders have one common disadvantage in that the protective jacket, even if it is only slightly damaged locally, has to be removed from the metal shaft of the electrode holder and a new protective jacket has to be applied which can cause lengthy production interruptions and high costs.
A further disadvantage of conventional electrode holders is the formation of slag and metal layers on the protective jacket of ceramic material, which can lead to disorders in arc furnace operation.
It was, therefore, suggested to create an electrode holder having a cooled metal shaft protected by rings of a material containing carbon, preferably graphite. Electrode holders of this general type have been employed and the protective carbon jacket mentioned has proved extremely useful. Graphite rings act as an excellent protective coat from the mechanical as well as the thermal viewpoint. One advantage of such a protective jacket is that, if it is partly damaged, the graphite ring in question may be exchanged, while complete removal of the jacket is only required if continuous protective coatings are used. A further advantage is that the formation of slag or metal layers is avoided, for due to the destruction of the graphite surface by oxidation they keep falling off the protective jacket. One disadvantage is, however, that in some cases the rings show a certain tendency towards cracking, believed caused by differing coefficients of thermal expansion of the protective jacket and the metal shaft constituting the electrode holder, and, consequently, by the resulting tensions within the protective ring.
Furthermore, there is one problem that all electrode holders described have in common, namely, how the metal parts can be fastened above the furnace lid. In general, this is achieved by means of mechanical clamping devices. Considering such factors as easy handling or quality of electric contact, it is advantageous to use such a mechanical clamp also as a means of current transfer. As a consequence, graphite or carbon moldings are usually used between the metallic part of the electrode and the clamping jaws of a bearer arm, for these moldings combine favourable current transfer properties with good mechanical and thermal properties.
However, the way how these moldings are to be fastened to the electrodes poses problems, as the moldings may break due to the clamping forces required. This may, in turn, lead to the loss of the moldings when, in varying the place of clamping e.g., the clamps have to be removed.